Magnetically self-shaping septum for beam deflection

ABSTRACT

A beam deflection septum magnet assembly for separating an extracted beam from a confined beam of a cyclotron. The separating member is a thin current-carrying sheet conductor that is self-shaping and assumes the same radius of curvature as the beam to be deflected when the end points are properly adjusted. The maximum field reduction obtained is limited mainly by the tensile strength of the septum and by the power that can be removed from the thin sheet conductor.

O United States Patent [1113,624,527

72] Inventor Ed 0. Hudson [56] References Cited UNITED STATES PATENTS [21] 72387 3 582 700 6/l97l H d 328/234 x [22] Filed sepn 15,1970 en ry [45] Patented Nov. 30, 1971 Primary Examiner-Raymond F. Hossfeld [73] Assignee The United States of America as Attorney-Roland A. Anderson represented by the United States Atomic Energy Commission ABSTRACT: A beam deflection septum magnet assembly for separating an extracted beam from a confined beam of a [54] MAGNETICALLY SELF-SHAPING SEPTUM FOR cyclotron. The separating member is a thin current-carrying BEAM DEFLECTION sheet conductor that is self-shaping and assumes the same 9 Claims,4 Drawing Figs. radius of curvature as the beam to be deflected when the end points are properly adjusted. The maximum field reduction [52] U.S. Cl 3322842331; obtained is limited mainly y the tensile strength of the septum [51] Int Cl 6 13/00 and by the power that can be removed from the thin sheet 501 Field ofSearch. 313/62;

FLU l D COOLED POWER LEADS t ADJUSTING SCREW O 3 3 l 5 a 11 l6 v FLEXIBLE CONDUCTOR 2 2 ll '2 4 LINK 9 fl j 14 1 PIVOT 13 COMPENSATING POINT serum 4 COlLS PATENTEB NUVBOIQYI 3624-527 sum 1 UF 2 MAGNETIC FIELD IN MAGNETIC FIELD T BOUNDARY BE I I IN VENTOR. Ed 0. Hudson BY w 4% ATTOR NEY.

PATENTEU unvao ISYI SHEET 2 [1F 2 zahmum INVENTOR. Ed D. Hudson ATTORNEY,

MAGNETICALLY SELF-SHAPING SEPTUM FOR BEAM DEFLECTION BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

The first extraction element in most extraction beam systems is an electrostatic deflector and this may be followed by some form of magnetic channel, when one is required. If large extraction efiiciencies are to be achieved, the septum (the element of the deflector separating the extracted beam from the circulating beam) must be thin and properly shaped and positioned. In fixed-energy machines and in cyclotrons with programmed orbits, and operating at fields below saturation, the beam path is fixed. However, in variable-energy, variable-particle cyclotrons, such as the Oak Ridge lsochronous Cyclotron (ORIC), the optimum shape may vary substantially. The septum in this case is made to be mechanically movable to accommodate these desired shapes.

In order to better understand the ORIC system in which the present invention was developed to be utilized, as well as for other systems, a description of this system will now be set forth.

The cyclotron is a device for accelerating charged particles to high energy without the use of correspondingly high voltages. For example, protons have been accelerated in ORIC to about 65 million electron volts (Mev.) with a peak dee potential of only 70 kilovolts. This is accomplished by causing the ions to follow a path that brings, them through an alternating electric field many times-adding a small increment of energy at each gap crossing. Ions are formed near the center of the vacuum tank by electron bombardment of gas molecules. They then travel outward in a spiral path determined by the electric and magnetic forces acting on them. Ions moving in a uniform magnetic field travel in a circular path whose radius is proportional to the momentum of the particle. In the cyclotron the circular path is converted to a spiral by the addition of energy from the electric field at each revolution. The magnetic field acts on the particles all the time they are in the cyclotron; however, the force from the electric filed is experienced only during the two short intervals of each revolution during which the particles cross the dee gap. After each gap crossing the voltage is reversed so that it will be of proper polarity to accelerate the particles at the next gap crossing. When the particles reach the maximum energy and orbit radius, they are electrostatically deflected into a region of reduced magnetic field which permits them to escape the influence of the magnet and travel through the evacuated beam pipe to a target.

The magnetic field of ORIC has pronounced azimuthal variations, derived from the sector-shaped pole tips; that is, the particles alternately experience highand low-magnetic fields along their paths. The transition regions between the highand low-magnetic fields, acting as magnetic lenses, produce focusing forces which constrain the beam to a narrow region near the median plane. Particle groups in many orbits may be accelerated at the same time if all groups reach the dee gap simultaneously. At very low energies a uniform magnetic field is needed, but for the energies attained in ORIC a radially increasing magnetic field is needed because of the relativistic increase in mass. The pole tip shape also causes the average field to increase with radius, keeping the ions in synchronism with the accelerating voltage as they gain mass relativistically.

To accommodate ions of different charge-to-mass ratios and to permit variable-energy operation, the frequency of the accelerating voltage must be varied along with the magnetic field intensity and shape. The main coils control the general level of the field; the valley and harmonic coils, located in the valleys of the sectored pole tips, control the azimuthal variations; and the trimming coils control the radial shape of the field.

The magnetic field and the radiofrequency can both be varied to provide for the acceleration of various ions to a wide range of energies as indicated above. The radiofrequency is stable to within 0001 percent and the RF voltage is stable to within 0.3 percent. The extraction efficiency is 60 percent for most beams. Extracted beams, in excess of I00 microamperes have been obtained, but most operation uses only 5 to 10 microamperes. The radial and axial emittances for 34-Mev. deuterons were measured to be 55 and 44 millimeter-milliradians, respectively. The energy resolution of the extracted beam varies from 0.3 to 0.5 percent. The resolution following the 153 analyzing magnet is 0.1 percent.

The ORIC is a powerful research tool; the acceleration of any ion, with an e/m ratio from I to la, to a desired energy makes possible a wide range of experiments. The variable energy and particle type provide good opportunities for study of reaction mechanisms and nuclear structure. Nuclear structure information obtained from direct reactions is being used extensively for the study of nuclear models. Particular emphasis has been placed on the study of the shell model by use of stripping and pickup reactions. Collective excitations of nuclei are studied with inelastic scattering. isobaric analog states are studied with charge-exchange reactions. By using polarized protons, the spin-orbit term of the nuclear interaction is investigated. The detailed structure of very light nuclei is also studied to better understand nuclear forces. Neutron-deficient radioactive nuclides are produced and studied either in situ (if the half-life is short) or after removal. Study of the radiations with alpha-, beta-, or gamma-ray spectrometers leads to a greater understanding of nuclear energy levels and decay schemes.

The facility is well instrumented so that complex nuclear particles can be studied. Reactions resulting in the emission of charged particles are studied with high-resolution solid-state detectors and a broad-range mass spectrograph, which has given a resolution of 1 part in 2000. Neutron energies are measured by time-of-flight techniques. The ORIC is also used in the study of radiation damage to various substances and to determine the effect of ionizing radiation on animal life. By simulatingthe radiation of outer space, data pertinent to the space program is obtained. Measurements of charged particle, neutron, and gamma-ray production cross sections aid in shielding studies. In addition to these areas of research, unusual opportunities exist in the fields of Coulomb excitation, heavy-ion physics, fission studies, solid-state physics, and atomic physics.

Cylotron Characteristics Magnet Data Field configuration 3 sector Pole diameter 76 in. Magnet gap, hill 7.5 in. Magnet gap, valley 28.0 in. Field at max. excitation, center I7 kgauss hill 22 kgauss vallcy 7.5 kgauss Magnet excitation [.23X 1 0' amp-turns Trimming coils l0 pairs Harmonic coils 9 pairs Valley coils 3 pairs Radiofrequency System Frequency range 7.0-22.5 MHz Dee voltage I00 Kv., max. Input power 650 kw., max.

Additional details of the structure and operation of the ORIC system may be obtained from Nuclear Instruments and Methods, 18, I9, Nov. 1962, pp. 46-61, 159-176, 303-308, and 601-605. The details of the mechanical electrostatic deflector of the ORIC system are described in the Oak Ridge National Laboratory Electronuclear Division annual progress report, No. ORNL-3630, dated June I964, pp. 46-50. A problem often arises with the mechanical septum of the electrostatic deflector of such a system in that the optimum shape must be precisely maintained. A very slight protrusion of the septum along the entrance portion of the extraction region can cause the entire exiting beam to be lost due to striking the septum, The present invention was conceived to substantially overcome the above problem in a manner to be described hereinbelow.

SUMMARY OF THE INVENTION It is the object of the present invention to provide an improved beam deflection device for a cyclotron, wherein the septum thereof can be automatically shaped during operation to match the beam path of the deflected beam from the cyclotron.

The present invention relates to an improved means for overcoming the above problem and accomplishing the above object. This is effected by providing a thin current-carrying sheet conductor as the septum of a magnetic channel which can be utilized to replace the electrostatic deflector of the ORIC system. The end points of the conductor are held fast and are properly adjusted as to position and the conductor assumes the same shape as the path of a charged particle as a result of forces induced in it by the magnetic field and the current flowing through the conductor. Thus, a magnetic channel with a septum as thin as usually found in electrostatic systems is provided by the present invention. An all-magnetic extraction system can operate at low voltages and can function in 7 smaller magnetic gaps than that required for an electrostatic system. The current in the magnetic channel can be regulated and controlled so as to maintain a precisely established deflected beam path.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram representing the forces on a thin current-carrying sheet conductor in a uniform magnetic field.

FIG. 2 is a partial view of the septum-magnet beam deflecting device of the present invention.

FIG. 3 is a sectional view along the lines 3--3 of FIG. 2.

FIG. 4 is a schematic view of the arrangement of magnetic compensating coils utilized in the device of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. I, the semicircular line denotes a current-carrying septum fastened at its end points, A and B to illustrate the principles of the present invention. Being thin, the septum is self-shaping under the action of the forces F, partially shown, which cause the septum to assume a proper and precise radius of curvature, provided only that the end points A and B are properly adjusted to some fixed positions. The forces F will be radially outward when the current flow through the septum is in the direction shown to decrease the field on the outside of the conductive septum which is a very suitable condition for a current-carrying sheet septum to be self-shaping.

The septum-magnet beam deflection device of the present invention is shown in FIGS. 2, 3, and 4, of the drawings, to which reference is now made. The fluid-cooled septum l of FIG. 2 is more clearly illustrated in FIG. 4 and it is 4 cm. wide and 0.3 crn. thick with the I-cm.-wide beam-slot center section being 0.075 cm. thick, for example. The septum l, in FIG. 2, is supported at each of its ends by a mounting assembly I] and 12, respectively, a cross section of the assembly I1 being illustrated in FIG. 3. The assembly 11 is permanently affixed to a plate member only partially shown, and the assembly 12 is affixed to a plate member 20, the members 20, 20 being pivotable about a pivot point 9. A deflector current-carrying fluid-cooled conductor 2, similar in construction to the septum l, and its relative spacing from the septum l are clearly shown in FIG. 4. Actually, the conductor 2 is provided in two sections 2 and 2', as shown in FIG. 2.

A fluid-cooled power lead 7 is connected to a fluid-cooled compensating coil 3 which in turn is connected to the fluidcooled conductor 2. The conductor 2 is electrically connected by means of a flexible conductor link 13 to the assembly 11 by means of a nut 19. The link 13 is electrically connected to the fluid-cooled septum l by means, not shown. The other end of the septum l at the assembly 12 is similarly connected by means of a flexible conductor link 14 to the fluid-cooled conductor 2' which in turn is connected to a second section of a fluid-cooled compensating coil 3 and the coil 3 is connected to a fluid-cooled power lead 8 to complete the circuit. The cooling fluid may be any suitable liquid such as water, for example, or gas.

It should be noted that only the two sections of the coil 3 are shown in FIG. 2 which are adjacent to one edge of the conductors 2 and 2' of FIG. 4, it being understood that a second pair of coils 3, 3' are provided adjacent to the other edge of conductors 2 and 2' (see FIG. 4) and that the coils 3 and 3' are electrically connected in parallel to the power source, not shown. The current flowing through the coils 3 and the coils 3 is 2,000 amperes, respectively, and the current flowing through the conductors 2, 2 and the septum I is 4,000 amperes, for example.

A second pair of3-turn 255-ampere compensating coils 4 and 4 are positioned adjacent to each edge of the septum l as shown in FIG. 4, only the coils 4 being shown in FIG. 2. The coils 4 and 4' are also fluid cooled and they are connected to a source of power, not shown. 1

An adjusting screw 10 is provided between and fastened to the respective plate members, 20, 20' and this screw, acting in conjunction with the plates 20 and 20', will permit the FIG. 2. 20, 20 and their attached assemblies 12, 1 l to pivot about the pivot point 9, such that the septum 1 can be initially adjusted to a desired radius of curvature. Thereafter, the magnetic forces acting on the septum 1 will effect the self-shaping of the septum in the manner indicated above.

The septum 1 may be cooled in the following manner. Cooling fluid from a source, not shown, is connected to the fitting 15 which then flows through a channel 21 of the assembly 11 (see FIG. 3) then through the septum 1, through a similar channel 21 in the assembly 12, and then through a fitting 17 back to the fluid source.

Cooling fluid from a source, not shown, also flows through fluid-cooled power lead 7, through one section of coils 3 and 3' and through conductor 2 to a fitting 16 which is connected back to the fluid source. Also, cooling fluid from a source, not shown, flows through fluid-cooled power lead 8, through the other section of coils 3 and 3', and through conductor 2' to a fitting 18 which is connected back to the fluid source. It should be noted that the radial aperture or separation between the conductors 2, 2 and the septum I is about 2 cm. for example.

As can be seen in FIGS. 3 and 4, the septum 1 effects the separation of the deflected beam 5 from the circulating beam 6 of the cyclotron.

The septum I may be fabricated from tungsten, tantalum, copper, or any other suitable electrical conducting metal. However, copper will be used in the ORIC, for example, for the reason that it will better withstand the tension developed in the septum operating at such high currents even though the residual activity in the copper septum will be higher than it is for septums made from graphite for a given beam loss. It should be noted that the septum is usually initially shaped to fit a desired orbit calculated from field measurements.

The purpose of the compensating coils 3, 3 and 4, 4' in the device of FIGS. and 4 is for improving the uniformity of the magnetic field in the circulating beam region of the cyclotron. The operating voltage for the above-described device may be of a selected value of a few volts up to 20 volts, for example.

As pointed out hereinabove, a thin current carrying conductor will assume the same shape as the path of a charged particle if the end points are properly adjusted. The radius of curvature at each point along the conductor will be inversely proportional to the magnetic field in which the conductor is located. The radial force on a conductor in a magnetic field, FIG. 1, is given by: F=BI/98 10, where F is in kg./cm.; B in kg., which may very from 6 to 18 kg. in the ORIC; and I in amperes, which flows through the septum l of FIG. 2. The tension developed in the current-carrying conductor in a magnetic field is T=pF, where T is in kg., p in cm., and F in kg./cm.

It should be noted that tests were performed on copper conductor 0.3 by 1.2 cm. that was placed in a 2.54 cm. gap magnet where the field was maintained at 17 kg., the current in the test conductor was held constant at 500 to 750 amperes, while an air-driven cylinder was used to load the conductor until the desired tension was developed. The conductor was strengthened for 4 cm. from the support points with copper pieces so as to provide a rigid section that could develop enough torque to overcome the friction forces. The air cylinder was actuated for 3,000 cycles at l/mm. for a period of hours. The conductor and the flexible leads connecting the conductor to the power source appeared to be in good condition at the end of the tests. The radius of curvature was changed from 90 cm. to 180 cm. during each cycle of the test while the tension on the conductor was maintained between 180 and 230 kg. giving a tensile stress on the conductor of 700 to 900 kg./cm. When the stress exceeds 800 kgjcm, elongation of the conductor becomes excessive. The above tests indicate that a self-shaping septum, as described hereinabove, can be provided that will operate up to years in a cyclotron that averages one septum shape change per day, and that there will be no difficulty in initially shaping a 0.3-cm.-thick septum that will operate with a tensile stress up to about 800 kg./cm

The field reduction provided by the device of FIG. 2 when operating in a cyclotron and with a radius of 75 cm., for example, is about 1 kg. Protons at 53 mev. will be deflected equally well by a l-kg. magnetic field or by an electrostatic gradient of 100 kv./cm. of an electrostatic deflector. For energies above 53 Mev., the magnetic field equivalent to the l00-kv./cm. electrostatic gradient will decrease until only 0.5 kg. is required for the magnetic field reduction for 320-Mev., protons.

It should be understood that the device of HO. 2 could be operated without the compensating coils 3, 3 and 4, 4. However, their use is preferred to improve the uniformity of the magnetic field as mentioned hereinabove.

An example of an alternate construction for the septum 1 of FIGS. 2 and 4 would be to provide a thin sheet conductor according to the present invention and also having a bellows attached along the top and alongthe bottom edges of the septum to provide cooling and tension strength. Also, a strengthening member may be made a part of or put inside the fluid coolant tubes to provide for this strength.

It should be understood that the principles of the present invention could also be applied to electrostatic deflectors having a septum thickness relatively thin as compared to the abovedescribed septum to provide for self-shaping of the septum thereof. The current required to shape such a thin current sheet septum of an electrostatic deflector is only a few amperes; however, if the radial magnetic force on the septum is to exceed the electrostatic force on the septum by a factor of five to 10, the current required for flow through the septum will be to 100 amperes, depending upon the electrostatic gradient. For example, the electrostatic and magnetic forces are equal in a system operating at 100 kv./cm. when the septum current is 10 amperes and the magnetic field is 17.5 kg. Thus, in this situation the septum current should be at least 50 amperes to provide for the proper self shaping of the septum.

Thin magnet septums, as described hereinabove, will be more reliable than electrostatic, deflectors and should give a more stable deflected beam. They will provide a better method for extracting higher energy particles than with conventional electrostatic deflectors. The all-magnetic extraction system of the present invention can operate at relatively low voltages and can function in smaller magnetic gaps than that required for an electrostatic system, and the current in the magnetic channel can be regulated and controlled so as to maintain a precisely established deflected beam path. Furthermore, self-shaping septums will provide a method for automatically and continuously adjusting the curvature of the septum to match the circulating and/or deflected beam paths and thus increase the 'beam deflection efiiciency with results that are two to three times better than with conventional devices that have to be adjusted mechanically.

It should be understood that the present invention is not limited for use with the Oak Ridge lsochronous Cyclotron. For example, it may be utilized in heavy-ion cyclotrons, or in synchro-cyclotrons.

This invention has been described by way of example and it should be apparent that it is equally applicable in fields other than those described.

What is claimed is:

1. In a cyclotron device provided with a magnetic field and a beam deflection assembly for separating an extracted beam from the circulating beam of said cyclotron, the improvement comprising providing a thin current-carrying sheet arcuate conductor as the septum of said beam deflection assembly, means for adjustably holding fast the end points of said sheet conductor, means for passing a desired and selected amount of current through said sheet conductor, and means for providing a compensating magnetic field in the vicinity of said sheet conductor, whereby said conductor assumes the same shape as the path of a charged particle in said extracted beam from said cyclotron as a result of forces induced in said current-carrying conductor by the magnetic field of said cyclotron.

2. The device set forth in claim 1, wherein said septum is 4 cm. wide, 0.3 cm. thick, with a l-cm.-wide center section thereof having a thickness of 0.075 cm.

3. The device set forth in claim 2, and further including a second, two-sectioned, thin current-carrying arcuate sheet conductor mounted in parallel relation with respect to said septum conductor and at a selected distance therefrom and being connected in electrical series with said septum conductor.

4. The device set forth in claim 3, wherein said selected distance between said conductors is 2 cm.

5. The device set forth in claim 4, wherein said compensating magnetic field means comprises a pair of two-sectioned compensating coils mounted perpendicular to the respective ends of said second conductor and being electrically connected in parallel with said second conductor, and a second pair of two-sectioned, spaced-apart, parallel compensating coils mounted perpendicular to said septum conductor which is spaced between said second pair of coils, and means for passing a desired and selected second amount of current through said second pair of coils.

6. The device set forth in claim 5, wherein said septum conductor, second conductor, and first pair and second pair of compensating coils are fluid cooled.

7. The device set forth in claim 6, wherein said septum conductor and said second conductor are made from copper.

8. The device set forth in claim 7, wherein said current flowing through said first pair of compensating coils is 2,000 amperes, said current flowing through said series-connected septum conductor and said second two-sectioned conductor is 4,000 amperes, and said second pair of two-sectioned compensating coils are each a three-tum, 255-ampere coil.

9. The device set forth in claim 8, and further including means for initially adjusting said end points of said septum conductor to provide a desired arcuate path of the same shape as said extracted beam.

i t l l i 

1. In a cyclotron device provided with a magnetic field and a beam deflection assembly for separating an extracted beam from the circulating beam of said cyclotron, the improvement comprising providing a thin current-carrying sheet arcuate conductor as the septum of said beam deflection assembly, means for adjustably holding fast the end points of said sheet conductor, means for passing a desired and selected amount of current through said sheet conductor, and means for providing a compensating magnetic field in the vicinity of said sheet conductor, whereby said conductor assumes the same shape as the path of a charged particle in said extracted beam from said cyclotron as a result of forces induced in said current-carrying conductor by the magnetic field of said cyclotron.
 2. The device set forth in claim 1, wherein said septum is 4 cm. wide, 0.3 cm. thick, with a 1-cm.-wide center section thereof having a thickness of 0.075 cm.
 3. The device set forth in claim 2, and further including a second, two-sectioned, thin current-carrying arcuate sheet conductor mounted in parallel relation with respect to said septum conductor and at a selected distance therefrom and being connected in electrical series with said septum conductor.
 4. The device set forth in claim 3, wherein said selected distAnce between said conductors is 2 cm.
 5. The device set forth in claim 4, wherein said compensating magnetic field means comprises a pair of two-sectioned compensating coils mounted perpendicular to the respective ends of said second conductor and being electrically connected in parallel with said second conductor, and a second pair of two-sectioned, spaced-apart, parallel compensating coils mounted perpendicular to said septum conductor which is spaced between said second pair of coils, and means for passing a desired and selected second amount of current through said second pair of coils.
 6. The device set forth in claim 5, wherein said septum conductor, second conductor, and first pair and second pair of compensating coils are fluid cooled.
 7. The device set forth in claim 6, wherein said septum conductor and said second conductor are made from copper.
 8. The device set forth in claim 7, wherein said current flowing through said first pair of compensating coils is 2,000 amperes, said current flowing through said series-connected septum conductor and said second two-sectioned conductor is 4,000 amperes, and said second pair of two-sectioned compensating coils are each a three-turn, 255-ampere coil.
 9. The device set forth in claim 8, and further including means for initially adjusting said end points of said septum conductor to provide a desired arcuate path of the same shape as said extracted beam. 